Electrophotographic photoconductor

ABSTRACT

An electrophotographic photoconductor has a conductive substrate and a photosensitive layer on the substrate. The photosensitive layer contains a compound represented by the formula (I),  
                 
 
     wherein X is selected from the group consisting of a single bond, O, CO, and COO, each of R 1  and R 2  is independently selected from the group consisting of an optionally substituted aromatic hydrocarbon group, an optionally substituted aliphatic hydrocarbon group, a polycyclic aromatic ring represented by formula (Ia), and a heterocyclic ring represented by formula (Ia),  
                 
 
     wherein Y represents a residual group to form said polycyclic aromatic ring or said heterocyclic ring, and said substituent being selected from the group consisting of a hydroxyl group, a cyano group, a nitro group and a halogen atom. The resulting electrophotographic photoconductor exhibits reduced residual potential after charging and exposure to light, and therefore facilitates high quality imaging.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to an electrophotographic photoconductor (also simply called “a photoconductor”) used in various electrophotographic apparatus, such as printers and copiers. More specifically, the present invention relates to improvements in the material composition of a photosensitive layer having organic materials as the major components.

[0002] A photoconductor has a basic structure consisting of a conductive substrate and a photosensitive layer having a photosensitive function laminated on the substrate. In recent years, organic photoconductors, using organic substances as functional components serving for charge generation and charge transport, are being actively researched and developed. In view of flexibility in material selection, high productivity and safety considerations, organic photoconductors have been put to practical use in copiers and printers.

[0003] The photoconductor is required to maintain surface charges in the dark, generate charges upon receipt of light, and transport the generated charges. The known photoconductors include a so-called single-layer type photoconductor, having these functions in a single photosensitive layer, and a so-called function separated laminated-layer type photoconductor, having a charge generation layer that mainly serves to generate charges upon receipt of light and a charge transport layer that serves to maintain surface charges in the dark and transport the generated charges upon receipt of light.

[0004] In these days, such a function-separated laminated-layer type photoconductor is in the main stream. These photoconductors have a photosensitive layer formed by laminating a charge generation layer and a charge transport layer. The charge generation layer is prepared by coating a coating liquid containing charge generation material of organic pigment and a binder resin dissolved and dispersed in an organic solvent. The charge transport layer is prepared by coating a coating liquid containing charge transport material of an organic low molecular-weight compound and a binder resin dissolved and dispersed in an organic solvent.

[0005] However, organic photoconductors available at present do not satisfy all of preferabld characteristics of photoconductors. In particular, a photoconductor exhibiting high residual potential after charging and light-exposure causes deterioration of image quality, which raises a problem of lowered gradation and resolution in a printer or a copier.

[0006] A mechanism that causes the residual potential may be considered as follows. When a photoconductor is charged and exposed to light, the charges of the photoconductor surface decreases and the electric field in the thickness direction lowers. This results in less charge transport capability. Thus, charges accumulate at the boundary between the charge generation layer and the charge transport layer. Furthermore, charges accumulate in the charge transport layer, so that the surface charges are not thoroughly cancelled. Therefore, the charges that remain on the photoconductor surface cause the residual potential.

OBJECTS AND SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide a photoconductor that overcomes the foregoing problems.

[0008] More specifically, it is an object of the present invention to provide a photoconductor that exhibits lower residual potential and prevents deterioration of image quality to produce excellent images.

[0009] The inventors of the present invention have made numerous studies to solve the problem, considering the above mechanism, and found that containment of a specific electron-attractive compound in the charge transport layer eliminates residual charges at the boundary of the charge generation layer and the charge transport layer. Moreover, the presence of these electron-attractive compounds in the charge transport layer results lowered residual potential in the charge transport layer. The invention has been accomplished based on the finding.

[0010] A photoconductor of the present invention solving the above problem, comprises a conductive substrate and a photosensitive layer on the conductive layer, the photosensitive layer containing a compound represented by the formula (I) below,

[0011] wherein, each of R¹ and R² independently represents an aromatic hydrocarbon group optionally having a substituent, an aliphatic hydrocarbon group optionally having a substituent, or a polycyclic aromatic ring represented by formula (Ia), or a heterocycle also represented by formula (Ia), the substituent being selected from a hydroxyl group, a cyano group, a nitro group and a halogen atom, X in formula (I) representing a single bond, O, CO, or COO, and Y in formula (Ia) representing a residual group for forming the polycyclic aromatic ring or the heterocycle.

[0012] Preferably, a photoconductor of the invention comprises a photosensitive layer including a charge generation layer and a charge transport layer laminated in this order, and the charge transport layer contains the compound represented by the above formula (I).

[0013] More preferably, the aliphatic hydrocarbon contains from 1 to 10 carbon atoms.

[0014] The above, and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0015]FIG. 1 is a schematic sectional view showing an example of a basic construction of a photoconductor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] Now, the present invention will be described with reference to some specific aspects of embodiments of the present invention.

[0017] Referring to FIG. 1, a negative-charging function-separated laminated-layer type photoconductor includes a conductive substrate 1, an undercoat layer 2, and a photosensitive layer 3. Photosensitive layer 3 includes a charge generation layer 4 and a charge transport layer 5 sequentially laminated in this order. Although the invention will be described in detail referring to the example of the laminated-layer type photoconductor, the present invention shall not be limited by the description of this example.

[0018] Conductive substrate 1 functions as both an electrode of the photoconductor and also as a support for the other layers constituting the photoconductor. Conductive substrate 1 may have a cylindrical shape, a planer shape, or a film-like shape, and may be formed of a metal or alloy such as aluminum, stainless steel or nickel, or glass or resin that has been treated to give certain conductivity characteristics on the surface.

[0019] Undercoat layer 2 is formed of a layer containing resin as a major component or of an oxide film such as alumite. Undercoat layer 2 is provided, as necessary, to control charge injection from the conductive substrate into the photosensitive layer, to cover defects on the surface of the substrate, and to improve adhesiveness of the photosensitive layer with the substrate. A resin material for the undercoat layer may be selected from an insulative polymer such as casein, poly(vinyl alcohol), polyamide, melamine, cellulose, and a conductive polymer such as polythiophene, polypyrrol, or polyaniline, and a material in suitable combination of these polymers. Undercoat layer 2 may further contain a metal oxide such as titanium dioxide or zinc oxide.

[0020] Charge generation layer 4, serving to generate charges upon receipt of light, is formed by depositing organic photoconductive substance as a charge generation material in a vacuum, or by coating with coating liquid in which powder of charge generation material is dispersed in a resin binder. Charge generation layer 4 desirably generates charges with high efficiency and has favorable capability of injecting the generated charges into charge transport layer 5. More specifically, the charge injection is desired to be less dependent on electric field, and to be facilitated even under low electric field. The charge generation material may be selected from phthalocyanine compounds represented by formulas (II-1) to (II-6), azo compounds represented by formulas (II-7) to (II-24), anthanthron compounds represented by formulas (II-25) to (II-32), and derivatives of these compounds. The resin binder used in the charge generation layer may be selected from polyester resin, poly(vinyl acetate), polyacrylate, polymethacrylate, polycarbonate, poly(vinyl acetoacetal), poly(vinyl propional), poly(vinyl butyral), phenoxy resin, epoxy resin, urethane resin, cellulose ester, cellulose ether, and suitable combinations of these substances.

[0021] The content of the charge generation material relative to the content of the resin binder in charge generation layer 4 is in the range of 5 to 500 parts by weight, preferably 10 to 100 parts by weight with respect to 10 parts by weight of the resin binder. The film thickness of the charge generation layer is determined depending on the light absorption coefficient of the charge generating substance, and is generally controlled to be not more than 5 μm, preferably, not more than 1 μm.

[0022] Charge transport layer 5 includes charge transport material, resin binder, and a compound represented by the general formula (I) according to the present invention. Specific examples of the compounds represented by the formula (I) are given by the formulas (I-1) to (I-26).

[0023] The charge transport material may be selected from a hydrazone compound, a butadiene compound, a diamine compound, an indole compound, an indoline compound, a stilbene compound, a distilbene compound, and any suitable combination of these compounds. Specific examples of these charge transporting substances are given by the formulas (III-1) to (III-12). The binder resin used in the charge transport layer may be selected from a polycarbonate resins such as bisphenol A type, bisphenol Z type or bisphenol A-bisphenyl copolymer, a polystyrene resin, a polyphenylene resin, and any suitable combination of these substances. Specific examples of these binder resins are given by formulas (IV-1) to (IV-7). The content of the charge transport material relative to the content of the resin binder in the charge transport layer is in the range of 2 to 80 parts by weight, preferably 3 to 70 parts by weight with respect to 100 parts by weight of the resin binder. The film thickness of the charge transport layer is preferably held in a range of 3 to 50 μm, more preferably, 15 to 40 μm, so as to maintain a practically effective surface potential.

[0024] When a compound of the general formula (I) is added into the charge transport layer according to the present invention, the content of the compound is usually controlled in a range of 0.1 to 10 parts by weight, preferably 0.2 to 1.0 part by weight with respect to 100 parts by weight of the charge transport material. In the case of single-layer type photoconductor in which a photosensitive layer is composed of a single layer, the content of the compound of formula (I) is controlled in a range of 1 to 50 wt %, preferably, 5 to 20 wt % with respect to total solid component of the photosensitive layer.

[0025] Undercoat layer 2 or charge transport layer 5 may further contain an additive such as an antioxidant or an UV-absorbing agent as needed for improving chemical stability against environment and stability against harmful light. Specific examples of such additives are given by the formulas (V-1) to (V-45).

[0026] A content of the additives, when added to the charge transport layer, is in a range of 0.01 to 10 parts by weight, preferably 0.01 to 0.5 parts by weight with respect to 100 parts by weight of the charge transport material.

[0027] The photosensitive layer may also contain silicone oil or fluorocarbon oil for the purpose of improving flatness or lubricating ability of the formed film.

[0028] Additionally, a surface protective layer, not shown in FIG. 1, may be provided as needed for improving chemical stability against environmental gases and increasing mechanical strength. The surface protective layer is formed of a material exhibiting chemical stability and durability against mechanical stress, and is required to transmit light that is sensed by the charge generation layer with minimum loss.

[0029] A photosensitive layer in a single-layer type photoconductor of the invention necessarily contains the compound represented by the general formula (I) as well as charge generation material and hole transport material. However, other material and construction of each layer of the single-layer type photoconductor may be appropriately selected in the similar manner as in the above-described laminated-layer type photoconductor.

EXAMPLES

[0030] The present invention will be described in further detail with reference to some examples of the embodiments of the invention.

Example 1

[0031] An organic photoconductor was produced for evaluation of electrophotographic characteristics.

[0032] Initially, an undercoat layer having thickness of about 3 μm was formed on the outer surface of an aluminum cylinder, which is a conductive substrate, by dip-coating a coating liquid and drying at 100° C. for 30 min. The coating liquid for the undercoat layer was prepared by dissolving and dispersing 5 parts by weight of alcohol-soluble nylon (CM 8000 manufactured by Toray Industries Co., Ltd.) and 5 parts by weight of fine particles of aminosilane-treated titanium oxide in 90 parts by weight of methanol.

[0033] On the undercoat layer, a charge generation layer having thickness of about 0.3 μm was formed by dip-coating a coating liquid and drying at 80° C. for 30 min. The coating liquid for the charge generation layer was prepared by dissolving and dispersing 1 part by weight of metal-free phthalocyanine of formula (II-1), which is a charge generation material, and 1.5 parts by weight of poly(vinyl butyral) resin (S-LEC KS-1 manufactured by Sekisui Chemical Co., Ltd.), which is a resin binder, in 60 parts by weight of dichloromethane.

[0034] On the charge generation layer, a charge transport layer having thickness of about 17 μm was formed by coating a coating liquid and drying at 90° C. for 60 min. The coating liquid for the charge transport layer was prepared by dissolving 80 parts by weight of a stilbene compound represented by the formula (III-11), which is a charge transport material, 120 parts by weight of polycarbonate resin represented by the formula (VI), which is a resin binder, 0.5 parts by weight of a carboxylic acid ester represented by the formula (I-1), and certain appropriate quantity of the antioxidant agent represented by the formula (V-32) in 600 parts by weight of dichloromethane.

Example 2

[0035] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was replaced by the compound (I-2) in Example 2.

Example 3

[0036] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was replaced by the compound (I-4) in Example 3.

Example 4

[0037] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was replaced by the compound (I-6) in Example 4.

Example 5

[0038] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was replaced by the compound (I-7) in Example 5

Example 6

[0039] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was replaced by the compound (I-8) in Example 6.

Example 7

[0040] An organic photoconductor was produced in the same manner as in Example 1, except that the charge transport material used in Example 1 was replaced by the diamine compound represented by the formula (III-8) in Example 7.

Example 8

[0041] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was replaced by the compound (I-2) in Example 8.

Example 9

[0042] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was replaced by the compound (I-4) in Example 9.

Example 10

[0043] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was replaced by the compound (I-6) in Example 10.

Example 11

[0044] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was replaced by the compound (I-7) in Example 11.

Example 12

[0045] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was replaced by the compound (I-8) in Example 12.

Comparative Example 1

[0046] An organic photoconductor was produced in the same manner as in Example 1, except that the compound (I-1) used in Example 1 was not contained in Comparative Example 1.

Comparative Example 2

[0047] An organic photoconductor was produced in the same manner as in Example 7, except that the compound (I-1) used in Example 7 was not contained in Comparative Example 2.

Evaluation of Photoconductors

[0048] The electrophotographic characteristics of each of the Examples and Comparative Examples were evaluated by the method described in the following.

[0049] A specimen of a photoconductor drum was charged in the dark to −650 V while rotating the drum. Then, the charge retention rate V_(k5) was measured during 5 seconds after charging and rotation were stopped. Subsequently, the drum surface was continuously illuminated and the light energy irradiated during the surface potential decayed from −600 V to −300 V was measured as the sensitivity E_(½), and the light energy irradiated during the surface potential decayed from −600 V to −100 V was measured as the sensitivity E₁₀₀. In the same time, the surface potential when the irradiated light energy amounted to 2.5 μJ/cm² was measured as the residual potential V_(r2 5). The results of the measurements are given in Table 1. TABLE 1 sensitivity sensitivity residual retention rate E_(1/2) E₁₀₀ potential specimen after 5 s V_(k5) (%) (μJ/cm²) (μJ/cm²) V_(r25) (−V) Example 1 93 0.20 0.62 30 Example 2 94 0.19 0.63 33 Example 3 93 0.20 0.62 34 Example 4 94 0.20 0.63 34 Example 5 93 0.21 0.63 33 Example 6 93 0.20 0.64 33 Comp. Ex. 1 94 0.20 0.62 40 Example 7 94 0.26 0.74 40 Example 8 94 0.25 0.75 42 Example 9 93 0.24 0.76 43 Example 10 94 0.25 0.75 41 Example 11 93 0.25 0.75 41 Example 12 93 0.26 0.75 43 Comp. Ex. 2 94 0.25 0.75 46

[0050] As is apparent in Table 1, the residual potential has been effectively decreased in the Examples 1 through 6 as compared to Comparative Example 1. Examples 1 through 6 contain the compound represented by the general formula (I) in the charge transport layer according to the present invention, while Comparative Example 1 does not contain the compound. In the same way, the residual potential has been lowered in Examples 7 through 12, containing the compound according to the invention, as compared to Comparative Example 2, without the compound.

[0051] The above-described examples are photoconductors that use a phthalocyanine compound and are to be applied to laser beam printers. In addition to that kind of photoconductors, we have further confirmed in the actual machines that the residual potential also effectively decreases in the photoconductor to be applied to an analogue copier, the photoconductor for a digital copier, and the photoconductor for a facsimile machine, when the compound of the formula (I) is contained in the charge transport layer.

Effect of the Invention

[0052] As described so far, the present invention provides an organic photoconductor that exhibits effectively decreased residual potential and thus gives excellent image quality.

[0053] Having described preferred embodiments of the invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope or spirit of the invention as defined in the appended claims. 

What is claimed is:
 1. An electrophotographic photoconductor comprising: a conductive substrate; and a photosensitive layer on said conductive substrate, said photosensitive layer containing a compound represented by formula (I),

 wherein X is selected from the group consisting of a single bond, O, CO, and COO, each of R¹ and R² is independently selected from the group consisting of an optionally substituted aromatic hydrocarbon group, an optionally substituted aliphatic hydrocarbon group, a polycyclic aromatic ring represented by formula (Ia), and a heterocyclic ring represented by formula (Ia),

 wherein Y represents a residual group to form said polycyclic aromatic ring or said heterocyclic ring, and said substituent being selected from the group consisting of a hydroxyl group, a cyano group, a nitro group and a halogen atom.
 2. An electrophotographic photoconductor according to claim 1 , wherein said photosensitive layer comprises: a charge generation layer laminated on said conductive substrate; a charge transport layer laminated on said charge generation layer; and said charge transport layer containing said compound represented by formula (I).
 3. An electrophotographic photoconductor according to claim 1 wherein said aliphatic hydrocarbon contains from 1 to 10 carbon atoms. 